The present invention concerns a roll used in the manufacture of paper, for example a center roll in a press or a calender roll, with which the web is in direct contact and from which the web is detached. A coating has been formed onto a cylinder mantle of this roll. This coating is partly made of metal and also includes a ceramic material.
The present invention also concerns a method for the manufacture of a roll thereof.
As is well-known, in a press section of a paper machine, a rock roll is used which is made of granite. The popularity of granite is based upon its surface properties, which provide a controlled detaching of the paper web from the rock face. Moreover, granite withstands the wearing effect of a doctor as well.
However, granite has certain drawbacks. Being a natural material, its properties vary, and internal flaws in granite and its tendency to crack constitute a serious obstacle to its use in certain applications. Moreover, a granite roll is heavy, which increases the tendency of vibration of the structures. The weight of the rock roll is also reflected in the dimensioning of lifting equipment and of foundations of the paper machine.
Synthetic rock rolls are also known in the prior art, which are principally polymer-faced rolls in which rock powder such as quartz sand, has been added among hard rubber or polyurethane. Drawbacks of these rolls have been excessive adherence of the paper web to the roll face, as well as poor mechanical strength.
In the Valmet FI Patent No. 70,273, a press roll is described which has a surface layer composed of a mixture of metal powder and an inorganic substance. The function of the metal is to act as a binder agent and to increase the toughness of the roll coating. The function of the inorganic substance is to provide a wear-resistant face of suitable surface energy, because the surface energy of the roll face must be within certain limits in order that the detaching of the paper web from the face of the press roll can be controlled.
In a roll in accordance with the Valmet FI Patent Appl. No. 853544, a suitable surface energy has provided wherein that the metal component is stainless steel containing chromium, the proportion of chromium in the metal being 9-35%. A stainless steel that contains an abundance of chromium is a hydrophilic material (chromium increases the hydrophilicity). On the other hand, by means of the alloying of chromium, wear-resistant chromium carbides were obtained in the structure. Chromium also increases the resistance of steel to corrosion. In such an "alloy", the ceramic material is separated from the steel itself as a chromium carbide.